1. Field of the Invention
The present invention relates to a method and apparatus to improve performance of a packet data communications system using orthogonal frequency division multiplexing (OFDM) with multiple transmit/receive RF front-ends and antennas. More specifically, it relates to system architecture and algorithm that use multiple RF front-ends and antennas to achieve transmit/receive diversity and thus improve system performance in packet OFDM systems such as IEEE 802.11a and IEEE 802.11g systems.
2. Description of the Related Art
The wireless local area network (WLAN) based on packet OFDM technology has experienced significant growth in recent years. A typical such system is an IEEE 802.11a/g system.
In an IEEE 802.11 WLAN, multiple WLAN stations communicating between each other in a confined environment form an ad-hoc wireless network called Basic Service Set (BSS). A BSS can optionally contain an Access Point (AP) which usually acts as a bridge between the wireless LAN BSS and a wired local area network. Without the presence of an AP, the BSS operates in the ad-hoc mode where each wireless station abides the CSMA/CA (Carrier Sense Medium Access/Collision Avoidance) medium access protocol, i.e., it ensures that the medium is idle before transmitting. With the presence of an AP, the BSS operates in infrastructure mode where the AP coordinates the medium traffic on top of the basic CSMA/CA medium access protocol used by individual stations.
In a packet system like an IEEE 802.11 WLAN, a unit of transmission is a packet. Although the packet size may vary, the packet structure is usually fixed as shown in FIG. 1. The first portion of the packet is the physical layer preamble (PHY Preamble). The PHY Preamble usually contains a known signal pattern that is used by the receiver for packet detection and parameter estimation (i.e., channel, carrier frequency, etc.). The second portion of the packet is the PLCP (Physical Layer Convergence Procedure) Header. The PLCP Header contains information to configure the receiver for the reception of the subsequent data portion of the packet. Such information includes, for example, data rate (IEEE 802.11a/g supports 8 data rates ranging from 6 Mbps to 54 Mbps), packet data length, etc. The last portion of the packet is the MAC (Medium Access Control) Data. The data portion of the packet could vary in length which results in variable packet length.
FIG. 2 shows the IEEE 802.11a packet structure and its corresponding PHY Preamble, PLCP Header, and MAC Data sections. The same packet structure is also used for the OFDM mode of IEEE 802.11g. The PHY Preamble section of the packet is 161s long and consists of 10 short preambles and 2 long preambles. The short preambles are typically used for signal detection, AGC (Automatic Gain Control), coarse frequency offset estimation, and possibly timing synchronization. The long preambles are typically used for fine frequency offset estimation and channel estimation. The first OFDM symbol after the long preambles contains the SIGNAL field of the PLCP Header, which encodes the packet length and modulation type information. Once the SIGNAL field is correctly decoded, the receiver will properly configure the baseband signal processing path to receive the MAC Data section of the packet.
Wireless communication system performance depends heavily on the radio propagation environment. FIG. 3 shows a simplified two-dimensional radio propagation environment. Radio signal obstacles are represented as one-dimensional walls with certain transmission and reflection coefficients. The graph has one transmitter and one receiver marked as circles. The radio signal propagation environment between the transmitter and receiver is called a channel. Due to wall transmissions and reflections, multiple replicas of the original signal transmitted by the transmitter are received. The replicas have different amplitudes and arrival times as shown in the impulse response graph in FIG. 4 and the corresponding channel frequency response is shown in FIG. 5 (the Y-axis is in a logarithmic scale).
The curve in FIG. 5 has a significant dip in the middle which is almost 20 dB deep. Such behavior is called frequency selective multipath fading and is typical in multipath channels. In an OFDM system, data is modulated on narrowband subcarriers. For example, IEEE 802.11a/g uses 64 narrowband subcarriers over a 20 MHz range. Because of multipath fading, each subcarrier experiences a different channel frequency response (In FIG. 5, the channel frequency response on each subcarrier is represented as a circle). The subcarriers around the dip would experience very low channel gains resulting in data loss on those subcarriers.
There are different ways to mitigate the effect of multipath fading. One way is through frequency diversity where the data is spread across multiple carriers so that the deep fades on some of subcarriers can be offset by gains on other subcarriers. Another method uses spatial diversity to mitigate multipath fading. In this latter method, the transceiver uses multiple antennas (in the form of an antenna array) and RF front-ends and combines the signals from different antenna branches to mitigate multipath fading.
In FIG. 6, the channel frequency responses on four different antennas that are spaced one carrier wavelength apart (about 6 cm for 5.25 GHz carrier) are plotted. The channel frequency responses are different on the four antennas. Often times when there is a dip on one of the channel frequency response curves, there is peak on another curve. Transmit and receive spatial diversity algorithm can be used to combine the signals on the four antennas so that deep fades on some of the channels can be offset by high gains on other channels resulting in a combined channel frequency response (shown as the top curve in FIG. 6) that is superior than any individual channel frequency responses. It should be noted that aside from mitigating multipath fading, combining signals from different RF branches has intrinsic combining gain depending upon the number of RF branches used (assuming noises are not correlated across RF branches).